An Active Matrix Organic Light Emitting Diode (AMOLED) display has been widely applied due to its wide angle of view, good color contrast effect, high response speed, low cost and other advantages. However threshold voltage drift may arise as a result of non-uniformity and instability of a Thin Film Transistor (TFT) back panel in a process flow.
FIG. 1 illustrates an existing pixel circuit including a transistor T1, a transistor T2, a transistor T3, a transistor T4, a transistor T5, a storage capacitor C1 and an Organic Light Emitting Diode (OLED), and FIG. 2 illustrates a timing diagram of the circuit in operation.
When a scan signal Scan (n−1) of the (n−1)-th row is at a low level and a scan signal Scan (n) of the n-th row is at a high level, the transistor T1 and the transistor T4 are turned off, and the transistor T5 are turned on, so the transistor T2 and the transistor T3 with the mirror structure, are also turned off, so that a signal stored on the storage capacitor C1 is initialized by the transistor T5 using an initial voltage signal Vinit. When the scan signal Scan (n−1) of the (n−1)-th row is at a high level and a scan signal Scan (n) of the n-th row is at a low level, the transistor T1 and the transistor T4 are turned on, and the transistor T5 is turned off, so the transistor T2 and the transistor T3 with the mirror structure, are also turned on, so that an image data signal Data is transmitted to a gate of the transistor T2 through the transistor T1 and the transistor T3, and at this time the transistor T4 is turned on, so drive current, dependent upon the signal loaded to the gate of the transistor T2, flows through the OLED to drive it to emit light. The voltage of the signal loaded to the transistor T2 is Vdata+Vth3, where Vdata is the voltage of the image data signal Data, and Vth3 is the threshold voltage of the transistor T3, and the drive current flowing through the OLED is Ioled=(k/2)(Vdata−Vdd+Vth3−Vth2)2, where k is a constant, Vdd is the voltage of a high-level signal VDD, and Vth2 is the threshold voltage of the transistor T2.
Although the transistor T2 and the transistor T3, which constitute a current mirror, are arranged adjacent to each other on a substrate, it may be difficult to make their threshold voltage same due to a TFT parameter in a fabrication process, and it may be difficult to make the drive current same when the same image data signal is received due to the threshold voltage drift of either of the transistors, which may degrade a display quality.
In summary in the existing organic light emitting diode pixel circuit, it may be difficult to make the threshold voltage of two TFTs consisting a current mirror substantially the same, so it may be difficult to make the drive current substantially the same when the same image data signal is received due to the threshold voltage drift of either of the transistors, which may degrade a display quality.